Pressure problems with upper

[Religious Shooter]

I have a 20" DPMS 1x9 CM upper and a 17" DPMS 1x8 SS upper. Both uppers were assembled by the same smith. The 17" is well used while the 20" probably has less than a 1000 rds through it.

I was switching BCG's between the two uppers and shooting the same load. The 20's casings had pierced primers (5 for 5) while the 17's casing had pressure signs but did not have pierced primers.

I got home and I loaded an empty case with an uncrimped bullet. Loaded it long. I magazine fed the empty round in my 20" and the COAL didn't change. No sticky extraction.

I don't have any gauges or anything. Other than just downloading the rounds for the 20" or sending it out... what other options do I have? How do I go about finding out if something (if at all) is wrong with this upper?

[mscott]

What kind of load are you shooting (powder, primer, bullet, velocity)? Is temperature possibly a problem (it could be with W748). Why do you think something is wrong with the upper vs. ammo?

[Religious Shooter]

The load that I used was 26 grains of A2230- (book max on the NRA service rifle data from Accurate) with 55 Hornady FMJBT. CCISR with LC cases.

I've shot the same load out of the 17" for about year. Maybe 2000 rounds with no problems. Pressure signs with the 17" (normal). But pierced primers with the 20" (5/5). 3162 fps out of the 17". 3341(!) out of the 20". Data is from separate days.

I also shot 27.5 of BLC2 with the same components as above. The firing pin indentation of the primers on the 20" were being pushed up. I would say that they were on the verge of being pierced with this load. Pressure signs were present with the 17" but the firing pin indentation were all down. Shot the same day. 3088 fps out of the 17". 3138 fps out of the 20". Data was gathered on the same day. Within 1/2 an hour of each other.

I shot the same A2230 loads on a different day with the two uppers. I took the rds out of the same baggie. I had changed the 20"s gas block to an adjustable one (PRI). Got to 2/5 pierced primers out of the 20". No pierced primers with the 17". Shot within minutes of each other.

Probably low 90's... 5 shots for each "test".

Basically I think it is the upper since the same load works OK on the other upper. But I have three other uppers. When I get the time I'll shoot the A2230 load through them and see if the 17"'s result is actually the exception.

[Bear1142]

Your 20" gun may have a .223 chamber and not a .556 chamber. There is a slight difference with the .223 have a shorter throat lead that can cause an increase in pressure. Depending on the model, DPMS manufactures a lot of barrels with .223 chambers and not .556 chambers.

I would have a gunsmith run a .556 chamber reamer down your 20" and see what happens. It will either fix your problem or do nothing. At least you'll be able to cross one possible reason off your list.

Erik

[kurtm]

That charge is on the warm side for sure, but the quickest way to find out your quest is to swap the firing pins from 17" to 20" and repeat the test. Firing pins can and will do what you describe, the down side is that it doesn't take more than about 5-6 pierced primers to F.U. a firing pin, I think I would look at the pins very closely and see if it might be a pin problem and not a pressure problem. as for the .223 vs. 556x45 at the charge weight you are running it shouldn't be a problem, BUT it could be a factor! KurtM

[TRUBL]

Actually Erik.....that would be a NO NO in this case.....untill we know what he is using for a die.

I suspect that he is either using a Dillon or RCBS .223 remington die, not a 5.56 nato die. Can you even buy a 5.56nato die commercially? I personally never looked for one.

Look at your components.....use a chronograph. Are you maybe using small pistol primers???

Also.....did your gunsmith ream these chambers? did he set the head space? or was this done by the company that made the barrels (DPMS barrels are typically head spaced correct).

You may need some chamber gages in your future, if you don't have any......you should (IMHO). Also.....are you small base sizing your rounds and shooting in a nato chamber?? Not a good thing either. Only small base size in a .223rem chamber. Lots of questions......you get to figure out the answers.

Bottom line.....if you are using the correct components, sizing correctly for the chamber, and you pierce primers....no matter WHAT anyone tells you, you are running too hot of a load for that chamber, use less powder. This is why you also need a chronograph....who cares how much powder you use as long as you get the accuracy and velocity you want. What may work in one gun, may not work in another.

Edited September 2, 2008 by TRUBL

[bigbrowndog]

TRUBL, speak truth

Trapr

This post was full of good info that I decided really didn't need to be posted so I deleted it.

Edited September 2, 2008 by bigbrowndog

[Bear1142]

Ha! Apparently my advice was full of FAIL!

I rescind my ill-advised advice.

Erik

[TRUBL]

Hey Erik, tis ok.......you still shoot em darn faster than most!!!

And besides....there is truth in what you said....if he was using Nato ammo in a .223 rem chamber.

[Bear1142]

Tim,

It's all good man. I was just thinking off the top of my head and not in a dianostic "check this first" way.

There were other symptons to check before reaming the chamber. Thanks for the backup.

Erik

[sobrbiker883]

The load that I used was 26 grains of A2230- (book max on the NRA service rifle data from Accurate) with 55 Hornady FMJBT. CCISR with LC cases.

Book max usually isn't kosher with LC brass (which usually has a smaller case capacity than commercial).........

I'd knock 5%-10% off your load and work up to a nice shooting accurate load. And check the FP, as states it doesn't take too many pierced primers to f' up a firing pin's tip.

33XX fps is a little quick.........

Edited September 3, 2008 by sobrbiker883